The rainbow trout (Salmo gairdneri) will be used to study the effects of dietary protein on hepatocellular carcinoma (hepatoma) induction and development. Aflatoxin B1 (AFB1), a useful secondary carcinogen giving consistent results in laboratory animals, will be utilized to induce neoplasia in the fish. Rainbow trout are the most sensitive known experimental animals as regards carcinogenic properties of AFB1 and are similar to humans and nonhuman primates in their in vitro metabolic conversion of AFB1. This model provides a convenient test system in which to determine the effects of diet on tumor induction and growth. In addition, exposure of the trout eggs to AFB1 for a brief period of time will yield a population of latent cancer-bearing animals in which dietary parameters can be evaluated. The effects of dietary protein on AFB1 carcinogenesis and biochemical mechanisms through which these effects are exerted will be investigated by combining in vivo studies of tumor production in trout along with in vitro studies on the enzymatic activation and detoxification of AFB1, identification and quantification of AFB1 metabolities, and conversion of AFB1 to active bacterial mutagens.